As the sun gets low in the sky, its light is refracted through a thicker and thicker section of the atmosphere. Eventually, the atmosphere not only refracts the light but begins to disperse it. Redder colors towards the horizon, and greener near the sun’s top.

As the last bit of sun vanishes below the horizon, there is a fraction of a second when only the green portion is left.

For many years, the Green Flash was only observed visually and was thought to be a complimentary color after-image on the retina caused by the disappearance of the bright red sun below the horizon. Only after the invention of color photography was the Green Flash captured on film and its true nature understood.